1. Field of the Invention
The present invention relates to a DTMF (Dual Tone Multiple Frequency) tone signal generator mainly used in information communication devices.
2. Description of the Related Art
An example of a conventional DTMF generator is shown in FIG. 11. Sine waves are generated by two sine wave generators 101 and 102. These two sine waves are combined at a fixed ratio by an adder 110, and a DTMF signal is obtained by converting the added digital signal to an analog signal using a D/A converter 111.
The sine wave generators 101 and 102 respectively comprise clock (frequency) dividers 104 and 107, counters 105 and 108, and Read-Only Memories (ROMs) 106 and 109 storing waveform data for one cycle. The frequency dividers 104 and 107 divide an output signal (clock) of an oscillator (OSC) 103 based on externally provided dividing ratio 1 and dividing ratio 2. The counters 105 and 108 count based on the outputs of the frequency dividers 104 and 107, and data from the counters 105 and 108 is outputted to the ROMs 106 and 109 as addresses. The ROMs 106 and 109 output waveform data corresponding to the received addresses.
As shown in FIG. 12, a DTMF signal is made up of a low tone group frequency component and a high tone group frequency component. There is a constraint that the ratio of the high tone group frequency to the low tone group frequency must be within the regulations of the telephone line. With the DTMF generator of FIG. 11, it was not possible to freely vary the ratio of the signal levels of the two sine waves of the DTMF signal output to the telephone line. As a result, there was a problem that the permissible range for the frequency characteristic of a telephone line interface was narrow, increasing cost.
In addition, in a system of the related art, a D/A converter was required, and so the cost of LSI (Large-Scale Integration) for the DTMF signal generator was increased.
The present invention was conceived in view of the above described problems, and the object of the present invention is to increase the permissible range for the frequency characteristic required of a telephone line interface, and to lower the cost of a DTMF generator.
According to the present invention, the aforementioned objects are attained by a DTMF generator, comprising a first sine wave generator and a second sine wave generator for respectively generating sine waves of specified frequencies, and a combining circuit for combining and outputting outputs from the first and second sine wave generators at a specified ratio, where a combining ratio of the combining circuit is variable.
The present invention is preferably also provided with a PWM (Pulse Width Modulation) circuit for generating a pulse of a width corresponding to the output of the combining circuit, and a filter for removing unnecessary frequencies included in an output from the PWM modulator.